As it is generally known, labelling machines are used to apply labels to containers of all sort. Typically used with beverage bottles or vessels are tubular labels (commonly called “sleeve labels”), which are obtainable by:                cutting the web unwound from a supply roll into a plurality of rectangular or square labels;        winding each cut label to such an extent that opposite vertical edges of the label overlap; and        sealing (e.g. welding, or gluing) the overlapping edges, thereby obtaining a corresponding tubular label.        
A particular type of labelling machine is known, in which a tubular label is formed on a relative cylindrical winding body (commonly called “sleeve drum”), from which it is subsequently transferred onto a relative container, for instance through insertion of the latter within the tubular label.
This type of labelling machine basically comprises a carousel rotating about a vertical axis and defining a circular path, along which a succession of unlabeled containers and a succession of rectangular or square labels from respective input wheels are fed to the carousel.
The labelling machine shapes the labels in a tubular configuration and applies them onto the respective containers. Finally, the labelling machine releases the labelled containers to an output wheel.
More specifically, the carousel comprises a number of operating units which are equally spaced about the rotation axis, are mounted along a peripheral edge of the carousel and are moved by the latter along the above-mentioned circular path.
Each labelling unit comprises a bottom supporting assembly adapted to support the bottom wall of a relative container and an upper retainer adapted to cooperate with the top portion of such container to hold it in a vertical position during the rotation of the carousel about the vertical axis.
Each supporting assembly comprises a vertical hollow supporting mount, secured to a horizontal plane of a rotary frame of the carousel, and a cylindrical winding body, engaging the supporting mount in sliding and rotating manner with respect to its axis, and adapted to carry a relative container on its top surface and a relative label on its lateral surface.
Each winding body is movable, under the control of cam means, between a raised position and a fully retracted position within the relative supporting mount.
In the raised position, each winding body protrudes from a top surface of the relative supporting mount and is adapted to receive a relative label on its lateral surface from the label input wheel; in particular, the label is wrapped around the winding body such that the opposite vertical edges of the label overlap each other.
In the fully retracted position, which is reached at the container input and output wheels, the top surface of each winding body is flush with the top surface of the supporting mount, so that containers are transferred onto and from the carousel along the same transfer plane.
A number of different techniques may be used for sealing the overlapping edges of the tubular labels, e.g. ultrasound welding, as shown in the International Patent Application No. WO 2005/085073.
After the sealing of the overlapped edges of a tubular label, the movement of the relative winding body from the raised position to the fully retracted position produces the insertion of the relative container inside the label, making the so obtained labelled container ready to be transferred to the output wheel.
The labelled container is then generally fed to a shrinking tunnel (known per se and not shown), where shrinking and adhesion of the label to the external surface of the container is attained.
It must be borne in mind that the function of a label applied on a container is not merely informative. As a matter of fact, the label is not solely intended to bear information concerning the content of the container (e.g. composition, volume content, safety or toxicology data, best-before date and the like) but, possibly through the association with a specific design or trademark which customers have become well-acquainted and familiar with, it contributes to the recognisability of the product and, consequently, to its marketability and commercial success. As a consequence, it is highly desirable that labels be applied to containers with a consistently satisfactory accuracy and homogeneity.
However, labelling machines of the type described above generally form part of plants having a very high throughput, hence the number of tubular labels formed and applied per hour by a single labelling machine may be in the range 24,000-32,000.
Under these conditions, it is almost inevitable that some kind of malfunction affects at some stage the overall labelling process, hence the quality achieved is not fully compliant with the production requirements. A number of different undesirable flaws may be detected, such as lack of proper alignment of the label on the container surface, imprecise or only partial welding/gluing of the overlapping edges of the label, curling or partial tear of a label, etc.
For this reason, labelling machines of the type described above may further comprise a discarding station at which the labelled containers which do not meet the desired quality specifications are identified and, consequently, rejected.
In labelling machines handling heat-shrinkable tubular labels, the discarding machine may be arranged immediately downstream from the carousel and, therefore, upstream from the shrinking tunnel, or, alternatively, downstream from the shrinking tunnel itself. Smaller anomalies and flaws which are not easily detectable prior to the shrinking process shall generally become evident following the application of heat which triggers the shrinking, hence this arrangement may be preferable.
Clearly enough, when detection of a certain flaw becomes recurrent, i.e. when too high a number of labelled containers per unit of time is rejected, it becomes highly likely that a damage or malfunction is affecting some component of the labelling machine.
For the cause of the malfunction to be identified and, much desirably, removed, operation of the labelling machine must be interrupted, so that the different units may be inspected and maintenance or possibly replacing of the malfunctioning unit me be carried out.
As a consequence, the production cycle has to be interrupted and operation of the labelling machine cannot be resumed until the cause of the malfunction has been pinpointed and removed.
Maintenance operations can be particularly complex and time-consuming, especially on labelling machines comprising a large number of operating units. In a field where high throughput is so crucial, any downtime may therefore represent a significant economic loss for the company.
It is therefore highly desirable that the downtime caused by the need to maintain a consistently high quality and accuracy of the label formation and application process be significantly reduced and kept at a minimum.